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Drain the Rain on the Plane

This blog accompanies 1-2 minute videos (11-14) of www.GreenBuildingSolutions.org’s new educational videos on the perfect wall. They present Joseph Lstiburek, Ph.D., P.Eng, ASHRAE Fellow of BuildingScience.com, an international expert in building science. All 14 Videos are here.

In 2000 years, one of the most innovative concepts of construction involves rain control. Builders know this innovation as “flashing” – a design that uses a metal drain ledge and drip edge to escort rainwater away from the wall plane. Workers layer a wall with sticky building paper in a shingle like fashion over the flashing to drain the wall’s moisture, rainwater, etc. (See 1:23 video The “Perfect” Wall Part 11 – Flashing and Rain Drain)

Historically, rainwater drained between a cladding and building paper applied to the outside of a wall. The water would penetrate the facade, and drip down the tarpaper into the flashing system. Rainwater exits over a most important element: the drip edge. “It is the inner building expressing itself,” says Dr. Lstiburek. This drain plane can be a water barrier or continuous insulation both systems work “just fine”. (See :48 Video – The “Perfect” Wall Part 12 – Flashing Back in the Day)

Over the last 20 years, technology emerged to “peel and stick” the water barrier to a primed wall as a membrane. This drains water that penetrates facades. This is a go-to system when talking about high-rise buildings. Builders wrap this flexible sticky coating plastic moisture barrier into the openings used for windows. If windows leak, which, with time, they most definitely will, this membrane will help push the water to the outside of the wall instead of allowing it to seep through into the building itself. Builders apply continuous insulation directly over this configuration as in the picture (See 1:43 video – The “Perfect” Wall Part 13 – Draining Walls & Windows).

Rainwater drainage is a fundamental concept behind the perfect wall and flashing is a revolutionary go-to tool regardless of the cladding and layering behind the facade.